Heating element comprising lanthanum chromite and oxidation-resistant silicon compound

ABSTRACT

A HEATING ELEMENT WHICH IS ELECTRICALLY CONDUCTIVE FROM ROOM TEMPERATURE TO AT LEAST 2000*C. IN AN OXIDIZING ATMOSPHERE AND OVER A LONG PERIOD OF TIME, THE ELEMENT BEING FORMED FROM A LANTHANIDE CHROMITE WHICH IS COMBINED WITH A VARIABLE PROPORTION OF AN OXIDATIONRESISTANT SILICON COMPOUND.

United States Patent 3,730,911 HEATING ELEMENT COMPRISING LANTl-IANUMCHROMITE AND OXIDATION-RESISTANT SILI- CON COMPOUND Grard Aubin, Evreux,and David Yerouchalmi, Le Mesnil Saint-Denis, France, assignors toCommissariat a lEnergie Atomique, Paris, France No Drawing. Filed Oct.4, 1971, Ser. No. 186,425 Claims priority, application France, Oct. 5,1970, 7035867 Int. Cl. H01b 1/06 U.S. Cl. 252--516 1 Claim ABSTRACT OFTHE DISCLOSURE A heating element which is electrically conductive fromroom temperature to at least 2000 C. in an oxidizing atmosphere and overa long period of time, the element being formed from a lanthanidechromite which is combined with a variable proportion of anoxidationresistant silicon compound.

This invention relates to a heating element in which the production ofheat is obtained by Joule effect resulting from the flow of an electriccurrent, said element being capable of opera-ting within a rangeextending from room temperature to temperatures above 2000 C. and evenmore especially in an oxidizing atmosphere.

Among the known applications of electrically conductive heatingelements, the most noteworthy are those which are designed in the formof Wires, cables, strips, plates, deposits and the like which arefabricated from metals such as molybdenum, tantalum, niobium, tungstenor from a metalloid such as graphite. These elements permit ofsatisfactory operation from room temperature to temperatures which canattain 2500 or even 3000" C. provided that work is carried out in aneutral atmosphere over a period of time which does not exceedapproximately twenty hours. Consideration has also been given to thefabrication of heating elements from a compound such as silicon carbidewhich is formed by means of special binders and permits of satisfactoryoperation in time and in an oxidizing atmosphere but only on conditionthat the temperature does not exceed 1600 C. Other compounds such asmolybdenum alloyed with a given percentage of molybdenum silicide permitof operation up to a maximum temperature of 1750 C. with a certaindegree of safety. Finally, it has already been proposed to make use ofcertain refractory oxides and especially limestabilized zirconium oxide,yttrium oxide, magnesia or different rare earths or thorium oxide(thoria). There is also a major drawback to be taken into considerationin this case since refractory oxides of this type are electricallyconductive only after preheating to approximately 1200 C. This does notpermit continuous operation of the heating element above roomtemperature and entails the need for more conventional ancillarycircuits which call for protection as soon as the temperature reachesthe threshold value of 1200 C. at which the element proper comes intooperation.

An appreciable advance in the technology of electrically conductiveheating elements has consisted in employing as constituent material ofsaid element either chromium oxide combined with alumina oralternatively chromites of lanthanides which may be mixed with zirconiaif necessary, the function of these compounds being to permit the flowof electric current from room temperature upwards when they are producedin the form of solids having very high density (87 to 99% oftheoretical), in which case their electrical conductivity is of theorder of 40 to 120 mhos/meter. Unfortunately, these ice compounds haverelatively high vapor pressures as soon as the temperature exceeds 1600"C., particularly in an oxidizing atmosphere in which the measuredevaporation rate is approximately one hundred times higher than in aneutral atmosphere and especially an argon atmosphere. This even holdstrue in the case of the most refractory and the most highly conductivechromite, namely lanthanum chromite which has a melting point ofapproximately 2450 C., although the addition of zirconia does have abeneficial effect in limiting to a certain extent the evaporation of thechromite which combines partially with the zirconia under the conditionsof use which have been mentioned in the foregoing. Moreover, thesedifferent compounds have relatively low thermal conduction and are thusvery delicate by reason of the thermomechanical stresses and especiallythermal shocks to which they are subjected when the temperature rises.If these compounds remain acceptable for use in spite of their highvapor pressure in an oxidizing atmosphere which results in rapid wear,it is in that case necessary to take very strict precautions wheninitiating either an increase or decrease in temperature; moreover, itis practically essential to fabricate the heating elements in the formof conductors having a very small cross-sectional area. These differentfactors make it difficult to employ these heating elements in practiceand result in capital costs which are usually of a'very high order.

The present invention overcomes the disadvantages of the varioussolutions which have been proposed heretofore by means of a heatingelement which is conductive from room temperature up to temperatures ofat least 2000 C. and permits utilization in an oxidizing atmosphere oververy long periods of time.

To this end, the invention consists in fabricating the heating elementfrom a chromite of the lanthanide series combined with a variableproportion of at least one silicon compound which exhibits oxidationresistance.

According to one preferred feature, the chromite of the lanthanideseries is lanthanum chromite whilst the silicon compound can be selectedfrom molybdenum silicon MoSi silicon nitride Si N silicon carbide SiC ora mixture of zirconium boride ZrB and molybdenum silicide MoSi Accordingto another preferred feature, the variable proportion of the siliconcompound is within the range of 5 to 20% by weight.

The invention therefore consists in combining with a highly refractorycompound such as a chromite of the lanthanide series of elements andespecially lanthanum chromite which is electrically conductive from roomtemperature to at least 2000 C. a suitable silicon compound whoseintended functions are to reduce to a considerable extent the vaporpressure of the chromite which is employed in particular in an oxidizingatmosphere, to increase the thermal conduction of the chromite andtherefore to make this latter less sensitive to thermal shocks withoutthereby modifying its oxidation resistance. The silicon compound ischosen so as to ensure that, during the process of manufacture, thegrains of the chromite which is employed are coated with a refractorysilica film which prevents vaporization of the chromite and isolatesthis latter from the surrounding atmosphere which can especially behighly oxidizing. The heating element thus formed can be produced withequal ease in the form of bars, strips, rods, hairpin elements, plates,cylinders and the like While being suited in each case to a particularuse and especially for furnaces of the tubular or parallelepipedal typeemployed in industrial laboratories, the susceptors of high-frequencycircuits or alternatively other thermal applications involving eitherintermittent or continuous operation.

The advantages of the heating elements which are produced in accordancewith the invention are essentially as follows: these elements areelectrically conductive from room temperature to a temperature which ishigher than 2000 C. and consequently dispense with the need forancillary circuits in furnaces or the like having either zirconium orthorium oxide elements in which the use of the heating element can becontemplated only at temperatures above 1100 to 1200 C.; these ancillarycircuits are usually formed of silicon carbide and have to be protectedabove this threshold value in order to prevent melting. The heatingelements of lanthanide chromite alloyed with a silicon compound can alsoreadily be employed in an oxidizing atmosphere (also, of course, in aneutral atmosphere) for any length of time which may be necessary.Finally, the operation of the elements can be maintained at the maximumtemperature during these periods of time without any difliculty.

By way of indication, the manufacture of a heating element in accordancewith the invention involves the following steps: in a first step, themanufacture of lanthanum chromite is carried out in a solar or plasmafurnace starting from chromium oxide and lanthanum oxide which are mixedin stoichiometric proportions. The product obtained is then ground to apowder having a particle size between 1 and 5 microns to which is addedin adequate proportions within the range of 5 to 20% a silicon compoundwhich is preferably selected from the products referred to earlier. Themixture which is thus formed is then finally shaped in a mold within anisostatic press under a pressure within the range of 1.5 to 5 t./cm. inorder to obtain a part having a density which varies between 90 and 95%of theoretical. The final step consists in carrying out a baking processwithin a gas furnace in a slightly oxidizing atmosphere with alevel-temperature stage at 1650-1700? C. for a period of one to twohours; during this step, the partial conversion of the silicon compoundto silica results in coating of the lanthanum chromite grains. Heatingtests are than carried out with a temperature rise in successive stagesof 100 to 200 C. per hour with a longer stage at 1600 C.

It must clearly be understood that the invention is not limited in anysense to the examples which have been more especially contemplated inthe foregoing but extends on the contrary to all alternative forms.

What we claim is:

1. A heating element having reduced vapor pressure I and improvedthermal conductivity and electrically conductive from room temperatureto at least 2000 C. in an oxidizing atmosphere and over a long period ofoperation comprising a mixture of lanthanum chromiteand from 5 to 20% byweight of at least one oxidation-resistant silicon compound selectedfrom the group consisting of molybdenum silicide, silicon nitride,silicon carbide and a mixture of molybdenum silicide and zirconiumboride.

References Cited UNITED STATES PATENTS 3,475,352 10/1969 Anthony et a1252-521 X 3,531,421 9/1970 Foex et a1. 252-521 X 3,321,727 5/1967Schrewelius 252-516 X 3,171,871 3/1965 Watson et a1. 252-516 X 3,490,0671/1970 Foex 252-521 CHARLES E. VAN HORN, Primary Examiner US. Cl. X.R.

